In the general aviation field prior to the early 1960's, the vacuum systems which powered gyros were driven by pumps which were lubricated by oil and referred to in the art as wet pumps. In the 1960's, the oil lubricated, or wet vane vacuum pumps, were replaced by dry vacuum pumps constructed of carbon vanes and rotors which were self-lubricating.
To this present day, the standard dry vacuum pumps in the market comprise mechanical carbon rotors and vanes operating in a hardened metal ellipsoidal cavity. These pumps are very unreliable and they fail without indication in a very erratic pattern. Wear of the carbon parts sliding back and forth and their basic lack of structural strength have been major problems since they came on the market over 20 years ago. Powdered graphite and carbon with various organic binders which are compressed under high pressures and baked, produce a mass which has very little tensile strength.
One of the principal problems with carbon pumps has been the catastrophic failure of the rotor. The general concept of utilizing a metal rotor for strength with carbon end plates, is old in the art as taught by U.S. Pat. Nos. 4,570,316; 4,198,195; 3,282,222; 4,050,855 and 3,552,895. Some of these designs have attached the carbon end plates to the metal rotor by molding the parts together, as illustrated in the first two patents mentioned above.
Another method is to attach the carbon end plate to the pump body so the end plates are stationary, which is exemplified in the last three patents mentioned above.